Injection molding systems powered by hydraulically driven actuators have been developed having flow control mechanisms that control the movement of a valve pin over the course of an injection cycle to cause the pin to move upstream and downstream during the course of upstream withdrawal of the pin from the closed gate position to fully open or during the course of downstream closure of the pin from fully open to a gate closed position over the course of an injection cycle in order to control the rate of flow of fluid material through the gate to correspond to a predetermined profile of fluid flow rates for the injection cycle that vary between a zero flow rate at gate closed position, maximum flow rate at fully pin open position and flow rates intermediate zero and maximum when the pin is in certain positions intermediate fully closed and fully open. In such prior systems a sensor can a condition of the fluid material or of the apparatus such as pin position and send a signal indicative of the sensed condition to a program contained in a controller that uses the signal as a variable input to control hydraulic drive fluid feed to the hydraulic actuator and in turn control movement of the valve pin in accordance with the predetermined profile of positions.